Polyurethane coatings are widely used for applications in which a protective overcoat or film is desired. For example, two-component or so-called “2K” polyurethanes containing a polyol or polyamine first component and a polyisocyanate second component will react when mixed to form a durable film containing polyurethane or polyurea linkages. Unfortunately, 2K polyurethane coatings can have lengthy drying times. If uncatalyzed, the coating can take hours to become tack-free and days to harden completely. The hardening rate can be accelerated by adding a suitable catalyst or initiator. Although a variety of materials have been suggested for use as initiators, nowadays polyurethane coatings typically are hardened using an organometallic compound such as dibutyltin dilaurate, e.g. as in U.S. Pat. No. 6,316,535 B1.
U.S. Pat. Nos. 3,347,804 and 4,256,848 disclose that zinc salts when used alone are very poor catalysts for polyurethane reactions. These patents (and U.S. Pat. Nos. 4,223,098 and 5,011,902) describe mixed catalyst systems containing a zinc salt of C2 and higher (e.g., C2-20, C2-21 or C2-22) carboxylic acids together with one or more other metal compounds. U.S. Pat. No. 5,156,915 describes a mixed catalyst system for polyurethanes based on certain ionizable zinc halide salts and bismuth-containing organometallic catalysts. Other zinc-containing catalysts for polyurethanes include those described in U.S. Pat. No. 4,478,959.
U.S. Pat. Nos. 4,517,330 and 5,319,018 describe acid-functional polymers reacted with transition metal compounds including certain zinc compounds. U.S. Pat. No. 5,610,232 describes an adhesive containing a carboxylate functional polyurethane whose isocyanate groups are reacted with water to carry out chain extension. The resulting carboxylate functional prepolymer is crosslinked via its carboxylate groups using zinc ammonium carbonate. U.S. Pat. No. 5,912,298 describes waterborne acid-functional polyurethane resins crosslinked via their acid groups using calcium compounds, and in a comparison example using zinc ammonium carbonate.